A typical, two-cycle, internal combustion engine includes a cast metal block defining a plurality of cylinders. With a loop charge system, a group of transfer ports or passages are associated with each cylinder and extend from the crankcase end of the block to the upper end of the respective cylinder. In operation of the piston, combustible charges are drawn upwardly from the crankcase through the transfer ports to the head end of the cylinder. The charges converge below the spark plug and the confluent charge is compressed by the upward stroke of the piston. After ignition by the spark, waste gases of combustion are expelled through the cylinder exhaust port with the downward power stroke of the pistons.
The conventional two-cycle engine block having a loop charge system is cast with a group of elongated slots, each of which extends through the wall of the block and communicates with a cylinder. Separately cast port covers are connected to the block and enclose the slots. The inner surface of each port cover defines the outer wall of the respective transfer port.
To assemble the port covers with the cast block, the mating surfaces on the covers and block must be machined, and holes are drilled in the covers and block and tapped. The mating surfaces are then gasketed and secured together by bolts. With a two cylinder engine, each cylinder has three transfer ports so that a total of six port covers must be assembled to the block, requiring approximately ninety separate components, such as bolts, gaskets, nuts, and the like. Thus, this assembly procedure involves a substantial labor and material cost and is a major contribution to the overall cost of the engine. Further, in order to provide adequate gasketing, the mating surfaces of the port covers and block must have a substantial width and this increases the overall weight of the engine.
Evaporable foam patterns made of a polymeric material, such as polystyrene, have been used in the past to cast metal components of internal combustion engines. In the evaporable foam casting process, a pattern is fabricated from the polymeric material and has a configuration conforming to the metal part to be cast. The pattern is then placed in a mold and the area around the pattern, as well as the cavities in the pattern are filled with an unbonded flowable material, such as sand. Molten metal is introduced to the pattern via a sprue, and the heat of the molten metal will vaporize the foam material, with the vapor being entrapped within the interstices of the foam, while the molten metal will occupy the void created by vaporization of the foam pattern to provide a cast metal part having a configuration conforming to the configuration of the pattern.